Friday, May 4, 2012

Murphy - My Consultant

Several weeks ago, I started designing a generic User Interface (UI) for my Propeller Microprocessor Projects (see previous post). The UI was planned to be an adaption of circuits that I am already using with current project (see previous post). The new adaptions included I2C Converters and stand-alone on-board 3.3V Regulator.

The UI board layout was sent for Manufacture on May 14 and was just received a few days ago. I was anxious to load a board with parts. My initial strategy was to load a board with as few parts as necessary to do some initial tests. But once I started loading, I decided to load all of the parts, starting with simple resistors and capacitors (what could go wrong? :-). Prior to installing the Major Expensive Components, I checked the LED circuits and did some simple resistant test - ALL looked good.

Then, In steps Murphy and Assumptions:

When I went to my parts storage bins for the 3.3V regulator, I could not find a SOT-89 3.3V regulator (similar to my often used L78L05 5V SOT-89 regulator). Maybe I have never had one, or maybe they do not exists with the foot print I used in the layout. But my board, as designed, defiantly needs one. When I laid out the board, I assumed they were in the bin. Now I will need to check availability and order something that will work.

When I went to my parts storage bins for the MCP23017 Dual I2C I/O Expander (16 pins of GPIO), I could not find them. I know they exist, and I thought I had order several, but I could not find them. My board as designed needs two. For this design and layout, I decided to use the Dual I2C I/O Expanders (similar to the previous used single expanders). They require less space for the available output pins, and they require one less device address on the I2C bus. So, now I need to order even more parts, to finish the build.

Prop UI

Most of the parts are installed, on the new UI board and is ready to test, but now must wait for the last few parts to be ordered - Thanks Murphy and Assumptions !

This iteration of the UI board was never planed to be the final form, it was just a functional test. I knew some modification maybe necessary. But that is the way it is, when building without breadboarding first.

UPDATE
I found the SOT-89 3.3Volt Regulators at Mouser, and they will be on order soon, along with a few other parts.